The invention relates to decoding received data that includes a sequence of data which are transmitted through different branches of a carrier radio interface for the purpose of error reduction, and, undergoing error reduction, which are combined to form a sequence of data to be output, wherein combining is implemented on the basis of carrier information values together with disturbance information values of the different branches relative to each other.
While signals or data are being transmitted via a radio interface with multi-path channels subject to fading, highly attenuated carriers (deep fades) are interfered with by superimposed errors. In order to avoid or reduce errors, diversity techniques are employed based on the principle of using independent channels since the low probability that all channels will be subject to simultaneous fading significantly enhances the reception of error-free data or recoverable data. The various diversity techniques serve to provide independent channels by differing means. Diversity significantly improves the required signal-to-noise ratio (SNR) for a specified bit error rate (BER) and allows for a considerably higher time variance while taking into account the maximum Doppler frequency—an important factor for reception by mobile receivers. In addition, diversity may, for example, be employed for synchronization schemes, such as carrier offset estimation, which can be operated with high accuracy, even given a low or negative signal-to-noise ratio.
One example of a diversity technique commonly employed is frequency diversity. Here the information or data are transmitted on different carrier frequencies. In order to ensure independent channels, the carrier separation must exceed the coherence width. Another diversity technique is time diversity. This is based on transmitting information distributed over different time slots. This can be achieved, for example, by employing interleaving combined with coding. However, a high level of cost/complexity is required for deep interleaving, with the result that this technique is not suited for applications having a restricted allowable delay. Yet another known commonly employed diversity technique is space diversity or antenna diversity. In space diversity, multiple antennas are employed, either on the transmitter side or the receiver side. In order to ensure independent channels, the antennas must be separated by several wavelengths.
The requisite combining of information from two or more receiver branches may vary in terms of the combining site and the combining method. A distinction may be made, for example, between combining before acquisition (i.e., before demodulation) of a received signal, and combining after acquisition (i.e., after demodulation). In Orthogonal Frequency Division Multiplex (OFDM) systems, combining is implemented after acquisition, usually directly after a Fourier transform before the equalizer or after a software-based combining (soft decision), that is, after demapping.
In addition, a distinction can be made between selection combining, equal gain combining, and maximum ratio combining. In selection combining, it is simply the branch or path with the highest signal-to-noise ratio that is selected. Implementation is very efficient. However, the information from all the other channels is dropped.
In equal gain combining, the signals with all weightings of the branches are set to a unit measure then added. This diversity technique is simpler than maximum ratio combining. However, the technique results in suboptimum efficiency. Maximum Ratio Combining (MRC) employs spatial combining weightings which are selected to maximize the signal-to-noise ratio of the output signal.
In order to achieve the maximum signal-to-noise ratio, the diversity branches must be weighted in a maximum ratio combiner by their corresponding fading amplitudes, and the phase shift of the channel must be compensated. The resulting sum must be normalized, thereby yielding the following MRC output value:MRC=(a1R1ejΦ1+a2R2ejΦ2)/(a12+a22)where ak is the amplitude, and Φk is the phase of the channel transfer function of the branch k for an instantaneously received carrier R.
These methods are generally known, from European Patent EP 1 125 377, for example, with respect to MRC combining for OFDM systems in a digital television system (e.g., Terrestrial Digital Video Broadcasting-DVB-T). Here, however, a technique is used which utilizes weighting, addition, and division. Interference is not taken into account. Only the channel transfer function and/or soft information is used. U.S. Pat. No. 6,151,372 discloses a method in which weighting, addition, and division processes are employed, an orthogonal detection being used having separate analog-to-digital converters for the I-component and Q-component.
However, these diversity methods are disadvantageous. With combining before the Fourier transform, optimization is not possible for each carrier. In software-based combining, use of channel state information (CSI) is not possible during this step. In carrier-based combining, conventional weighting, addition and division methods are employed—with the resulting complexity/expense. In addition, it is not possible to use interference information. To the degree that these methods take into account a noise level, this level is assumed to be constant for all carriers.
Therefore, there is a need for an improved technique for processing data which are transmitted through a radio interface by different channels.